1. Field of the Invention
The present invention pertains to the transfer of data signals point to point. More particularly, the invention relates to transfer of data between source and receiver wherein the data in the form of an electrical signal is converted into light waves, transferred via a fiber optic cable to a receiver and then converted back into an electrical signal.
2. Description of the Prior Art
Transmission of multiple data signals from one point to another can be accomplished in a virtually unlimited number of ways. Transfer of information can occur over different frequencies (optical, microwave, & radio) and media including air, twisted wire, coax and more recently, fiber optic cables. Examples of data transmission in air include television and radio. Transfer of information by wire could include, for example, macroscopic structures such as coax or twinax cable for carrying a television signal from a receiving antenna to a television set or a cable connecting a peripheral unit such as a printer to a personal computer, to microscopic structures such as the minute electrical paths that guide electrons in an integrated circuit. Examples of transmission of data using fiber optic cables include fiber optic telephone cables. In each of these types of systems it is typically desirable that data should be transferred between source and receiver as quickly as possible and with utmost accuracy and maintenance of desired signal characteristics.
Two broad categories of transmission of data would include transmission by analog and digital means. Within each of these types of transmission, numerous modulation techniques of data have been used including FM, AM, and pulse modulation (PM). Techniques providing means for not only modulating a carrier with information from one signal but for multiplexing numerous signals for transmission together include time division multiplexing (TDM), and frequency division multiplexing (FDM). FDM in essence involves stacking a number of data channels side by side in the frequency domain to form a composite signal. The composite frequency multiplexed signal is used to modulate a carrier in a conventional manner. TDM is a derivative of pulse modulation and involves interleaving in time the narrow pulses of several pulse modulated signals to form one composite signal. Separation of the TDM pulses at the receiver is accomplished by directing the pulses into individual channel filters.
In recent years, as a result of the maturation of fiber optic technology in transmission systems, advances have been made in a multiple carrier technique referred to as wavelength division multiplexing (WDM). This technique is the optical equivalent of frequency division multiplexing used in RF coaxial transmission. In WDM, each discrete data channel is modulated onto an optical carrier of a fixed wavelength and each of the carriers are then fed into the optical transmission medium. The individual carriers are recovered at the receiver by separating the carrier into its individual wavelength components. One such example of a WDM is disclosed in U.S. Pat. No. 4,926,412, the essentials of which are incorporated by reference herein. There, a WDM is disclosed having paraxial transmission optics.
A transmission optics WDM differs from a reflection optics system in that in the former, the light beam changes direction only once in the system; it is first collimated by a lens, diffracted by a grating, and then focused by the lens into the fibers. In paraxial optics, the input and output optical fibers are maintained close to the optical axis to limit losses in the system due to dispersion broadening, and image aberrations.
In a reflection optics WDM, the beam changes direction three times; the beam is first collimated by a mirror, diffracted by a grating located in roughly the same plane as the fiber ends, and focused by the first mirror into the fibers. Reflection optics WDMs, produced by Instrument SA and others, are based on relief surface holographic gratings, usually coated on curved surfaces that simultaneously perform imaging operations. Other types of WDMs, produced by Kaptron, for instance, utilize spherical imaging reflection surfaces, usually coated with multilayer dielectric films to provide wavelength selective reflectance (see, e.g., F. Unterleitner, Fiberoptic Product News, Vol. 5, No. 12, November 1990, p. 27). An entirely different WDM approach based on biconical coupler technology, is proposed by Gould Electronics. Still another WDM system, based on lens transmission optics and highly non-uniform flat volume holographic gratings, has been proposed by Physical Optics Corporation in the above mentioned U.S. Pat. No. 4,926,412. References relating generally to optical fibers and fiber communications are plentiful and include for example: S. Miller and I. Kaminow, Optical Fiber Telecommunications (Academic Press, 1988); and D. Baker, Fiber Optic Design and Applications (Reston, 1985)incorporated by reference herein.
Without question, transmission of data optically versus by wire means is becoming more and more prevalent. Numerous reasons exist for using optical transmission of data, as opposed to electrical transmission including bandwidth limitations, electromagnetic interference, weight and bulk. In order to achieve high bandwidth in an electrical data transmission system, the wires must have large diameters for shielding from EMI and consequently are bulky and heavy. Furthermore, power losses associated with data transmission over electrical ires are very large, and signal repeaters must be placed at relatively small intervals (500 m) even for low frequencies (&lt;10 MHz).
Transmission of data optically, on the other hand, can provide huge bandwidth characteristics, extremely low loss even over long distances and immunity to electromagnetic radiation even in environments saturated with electronics such as aircraft. Furthermore, because the bandwidth of an optical fiber actually increases with a decrease in the diameter of the fiber optic cable, huge bandwidths (&gt;300 MHz for 1 km length of multimode fiber) are obtainable with extremely light and nonbulky transmission lines.
Although the majority of LANs are still coax-based or "twisted pair", video transmission systems, which are more demanding than LANs because of higher bandwidth requirements and remote location desirability are better suited to fiber optics. Typical high speed fiber optics systems are red-green-blue (RGB), closed circuit television (CCTV), and computer aided design (CAD) RGB and CCTV are typically analog with a 10-50 MHz throughput while CAD is digital or analog with 120-300 MHz throughput. Typical multi-mode fibers used for data transmission have 50/125.mu., 62.5/125.mu., 100/140.mu., and 200/380.mu. core diameters. Fibers having these four core/cladding diameters are standard in most fiber optic applications and their cost of manufacture continues to decrease. Note that in a state of the art single-wavelength design only the core diameter is important because the core contains the traveling beam to the exclusion of the other portions of the fiber such as the cladding.
The advantages of optical transmission of data have not escaped industries where transfer of data is critical to transaction of daily business. Video local area networks (LANs), i.e., video conferencing, security systems, and the securities brokerage industry are three examples where transfer of information by fiber optic cable has been in place for sometime. Fiber optic cables have been used to transfer data between a video camera and a security alarm processor in high end security systems. In another application, information is transferred between a main computer that keeps track of market conditions and the tens of screens in a securities trading room.
In the future, Integrated Services Digital Networks (ISDN) which may provide 3-channel information for homes and businesses (telephone, video and data) will require high quality transmission. These systems, and Broadband Integrated Services Digital Networks (BISDN) will become commonplace.
In securities trading applications, typically called data feed terminal systems, the three video components (red, green, and blue), are continuously fed to trading room video screens in basically two ways. One means of transfer currently in use is inputting each of the three components of the video signal from the source computer into a light emitting diode (LED), or laser diode (LD) which converts each of the electrical signals to a light signal which is then fed, via a dedicated, separate optical path, to the trading screen. Another slightly newer and less common approach is to first multiplex (using TDM) the three electrical components of the video signal and then feed the multiplexed electrical signal to a LED, or LD for conversion to a multiplexed light wave which is then fed to the trading room video screen and reconverted to three electrical signals. Most such systems in use today are standardized around the RS 170, and RS343A standard for computer generated video signals.
The major drawback of the first system is that it requires the use of three separate modules in the electronics rack and, most importantly, requires three lengths of fiber optic cable to be run from the central computer to the trading room screens potentially many floors below. This obviously creates size and cost constraints. The major drawback of the slightly newer implementation of data transfer (TDM) is that the signals must be electronically multiplexed and then fed to a laser which must convert the multiplexed electrical signal accurately into a multiplexed light wave and transmit it to the video screen. Committing one light source to the task of converting a multiplexed electrical signal into a multiplexed light signal is less than desirable because it is typically a low efficiency conversion. The bandwidth of the fiber also becomes an issue in this format because 200.mu. core fiber cannot effectively transmit the bandwidth of an RGB signal on a single wavelength due to modal dispersion. Length is an issue as well; the signal can be broadcast but only over very short distances.
Still another disadvantage of TDM technique is that the multiplexed signals must be of the same modulation format, usually digital. To the contrary, WDM fiber-optic systems can multiplex, through various wavelength carriers, not only different format signals such as digital and analog, i.e., RS170 & RS232, but also various types of information related to different wavelengths, specific to a particular sensing medium such as in Raman spectroscopy, for example.
There are three basic types of light sources used in optical fiber data transmission, surface light emitting diodes (LEDs) and edge limiting LEDs (ELEDs), and laser diodes (LDs). Surface emitting LEDs have been in use for many years in many different applications. They are extremely reliable and relatively inexpensive. Laser diodes, on the other hand, are a much more recent technology, are slightly less reliable than LEDs, and are usually more expensive. LDs, however, as well as ELEDs have certain advantages over LEDs, that are becoming consistently achievable as LD technology matures in the compact disk (CD) industry. LDs are well known in use as the light source for reading CDs in now quite common CD players. The market for LDs created by the CD industry is large and has caused the development of standard LD wavelengths located in the 1st transmission spectral window: 750-850 nm. Within this range, Sharp has developed standard LD WDM wavelengths 750, 780, 810, 840, all in the vicinity of the CD wavelength 780 nm. Siemens, Phillips, Hitachi and Ortel also make LDs. Because of the huge production of LDs in these standard wavelengths, LDs have become extremely low cost ($10-$30) and price competitive with LEDs. Recently, ELED technology has achieved maturity, with a typical unit price of around $100. ELEDs' wavelengths, on the other hand, are usually located in the 2nd transmission window, around 1300 nm.
The primary advantage of LDs over LEDs is that LDs have much narrower spectral characteristics. Furthermore, LDs are much faster than LEDs. It is difficult to achieve 200 MHz with LEDs, while LDs can obtain greater than 1 GHz bandwidths. Additionally, the life of a typical laser diode is 250,000 hours or 120 years assuming it is not abused with high current or physically damaged. Also ELEDs have significantly narrower linewidths than surface-emitting LEDs, typically 50-100 nm versus 100-200 nm.
Unfortunately, with respect to transmission of multiple channels from source to receiver, TDMs and FDMs require very troublesome and sophisticated electronics while multi-fiber solutions are expensive and difficult to implement in space tight applications. Therefore, a data transmitting system that does not require the use of TDM or FDM multiplexing nor multiple fibers would be of great benefit and cost saving for all data transmission applications.